The present invention relates to a roll for use in a rolling process of non-ferrous metal as in case of producing a sheet of aluminum, copper, titanium, nickel and an alloy thereof.
When a sheet of non-ferrous metal is produced, for example, a sheet of aluminum (hereinafter referred to as "Al"), first of all, an ingot heated to 480.degree..about.520.degree. C. is prepared, and this ingot is subjected to a hot rolling process at a temperature range of 180.degree..about.320.degree. C. to produce a sheet. The sheet thus obtained by the hot rolling treatment is subjected to a cold rolling treatment to make its thickness a predetermined value, and then, it is cut to a required length in accordance with the aim and is subjected to a heat treatment. Then it is subjected to a chemical treatment, thereby obtaining a manufacturing product.
On the production line of the above Al sheet, the hot rolling and cold rolling processes are indispensable, so that various rolls are utilized in accordance with a working aim in these processes. These rolls used in accordance with the application have to possess the following attributes.
(1) Proper coefficient of friction should be shown for sheets (Al sheet). That is, (a) A preset tension can be given. (b) Slipping of sheets does not occur. (c) Preventing the meandering of the sheets. PA1 (2) Surface inperfections such as scratches and dents must not occur. PA1 (3) Fats and oils or the like, metal and nonferrous metal dusts should be avoided. PA1 (4) The above performances can be held for a long term. PA1 (1) In the elevated temperature (300.degree. C. or more), the chromium plated layer loses its hardness, thereby decreasing abrasion resistance thereof extremely. PA1 (2) When the chromium plated layer is used at the temperature of 220.degree..about.250.degree. C. for 1,500 hours, micro cracks present in the chromium plate become progressively large, so that surface imperfections are applied to the sheets, or the oxide coating of Al or particles of metal Al adhere to the micro cracks, resulting in an unsatisfactory operation of the roll. PA1 (3) During the cold rolling process, the rolls undergo a slip phemomenona, frequently so that with a frictional heat caused at that time. Al dust of the sheet adheres onto the surface of the chromium plated layer of the roll, resulting in a generation of the surface inperfections on the sheets. PA1 (4) As a countermeasure to the adhesion of Al dust or the like to the rolls as described above, the rolls are frequently treated to remove the adhered Al dust or the like with the use of abrasive papers. Such a maintenance for the countermeasures must be carried out frequently, so that it will require a great deal of labor. Moreover, during such a maintenance with the abrasive papers, the chromium plated surface of the rolls is susceptible of imperfection, so that there is a tendency for Al dust or the like to adhere at future time.
Hitherto, to achieve the foregoing, the roll was electroplated with coatings of chromium (chromium plating).
The above chromium plated roll has very excellent characteristics in the point that the surface inperfections do not apply blemishes to sheets, but the following problems exist.
As described above, the foregoing problems decrease productivity of Al product line, deteriorate quality of the sheets, and decrease the lifetime of the rolls.